To obtain an excellent high-temperature superconducting wire, it is necessary to form a superconducting film having high orientation on a substrate.
For this end, conventionally, with respect to the manufacture of an oxide superconducting wire, particularly, a Y-based oxide superconducting wire, there has been known the Y-based oxide superconducting wire which is manufactured as follows. As described in patent documents 1 to 3 and non-patent document 1, as an intermediate layer, an oxide layer such as a cerium oxide (CeO2) layer, a stabilized zirconia (YSZ) layer or an yttrium oxide (Y2O2) layer is grown on and is laminated to a biaxially-crystal-oriented metal substrate by an epitaxial growth using a sputtering method. Then, an oxide superconducting body layer such as the Y123 thin film is grown on and is laminated to the oxide layer by an epitaxial growth using a laser abrasion method or the like, and an Ag layer or a Cu layer is laminated to the superconducting body layer as a protective film (RABITs method).
It has been also known that, for the acquisition of excellent superconducting wires using the above-mentioned RABITS method, it is important that the above-mentioned metal substrate is highly biaxially oriented.
Further, such a superconducting body is applicable to AC apparatuses or AC applications such as cables, coils and magnets. In view of a state where the superconducting body is manufactured by a reel-to-reel method in order to form the superconducting body into a tape shape or an elongated shape or an intermediate layer or a superconducting body layer are formed as a film at a high temperature of 600° C. or more, the above-mentioned metal substrate is required to satisfy conditions that the metal substrate has high strength, and has various properties which are uniform in the longitudinal direction and the like.
Further, in a case where the superconducting wire is used in the AC cable application or is used in a high magnetic field, when the wire is formed of a magnetic body, a current loss becomes large when an AC current is used thus giving rise to a drawback that the superconducting property is deteriorated. Accordingly, it is necessary to make the above-mentioned metal substrate have weak magnetic property or non-magnetic property.
As a method for highly biaxially orienting the above-mentioned metal substrate, in general, there has been known a method in which the metal substrate is subject to cold rolling at a draft of 90% or more thus giving a large uniform strain to the whole material and, thereafter, the metal substrate is recrystallized by heat treatment thus obtaining the highly biaxially oriented metal substrate. Particularly, it has been known that Ni, Cu or an alloy of these metals exhibits the high biaxial crystal orientation.
Particularly, because of its high strength compared to Cu and its affinity with an intermediate layer and the like, Ni has been widely used from an early stage of development of the biaxially oriented substrate. However, there still remains a drawback that material strength of crystal-oriented Ni is low, that is, 30 MPa in terms of yield stress and, further, the biaxial orientation of pure Ni is approximately 10° in terms of a half value width (Δφ) of a φ scan peak (α=45°) in a pole figure by an X-ray diffraction which becomes an index.
Accordingly, to improve the biaxial crystal orientation of a pure Ni layer, there has been proposed a method (SOE method) in which NiO is formed by oxidizing a surface of the pure Ni layer at a high temperature so that the degree of biaxial crystal orientation is improved. However, there still remains a drawback that the biaxial orientation is approximately 8° in terms of Δφ or the method requires heat treatment which takes a relatively long time at a high temperature of 1000° C. or more and hence, the number of steps is increased and a cost is pushed up whereby the method is uneconomical.
Further, as the metal substrate, there has been proposed an Ni—W alloy where 3% to 9% of W is added to Ni, wherein the Ni—W alloy is developed by taking strength and biaxial crystal orientation into consideration. The Ni—W alloy exhibits the higher biaxial crystal orientation than pure Ni and the biaxial orientation is 7° or less in terms of Δφ.
The strength of the Ni—W alloy is improved compared to the strength of the pure Ni, that is, yield stress of the Ni—W alloy is enhanced to 195 MPa. However, the Ni—W alloy does not exhibit the sufficient strength necessary in handling during conveyance by a reel-to-reel method at a high temperature at the time of forming an intermediate layer as a film. That is, the handling of the metal substrate made of the Ni—W alloy is not easy.
Further, to secure the strength of the metal substrate, a thickness of the metal substrate cannot be decreased to a value less than 100 μm so that there arises a drawback that a cost cannot be decreased.
Further, Ni and Ni—W alloy are ferromagnetic and hence, when these materials are used for producing the metal substrate, due to the restriction that a thickness of the metal substrate cannot be decreased from a viewpoint of ensuring strength, a current loss becomes large in AC applications. Further, even when a superconducting body layer which possesses the favorable crystal orientation can be laminated to the metal substrate, the metal substrate cannot acquire sufficient superconducting property.
Further, the above-mentioned Ni—W alloy is not a popularly used material, is difficult to obtain, exhibits poor workability so that the manufacture of the substrate having a large width is difficult, exhibits poor productivity, and is expensive.
Further, as a material for forming a metal substrate which can overcome a problem on ensuring strength other than an Ni alloy, there has been proposed a clad material which is formed by laminating a metal core layer and an Ni alloy layer by cold drawing or by a cold rolling method (patent documents 4, 5, 6).
Patent document 1: Patent 3601830
Patent document 2: Patent 3587956
Patent document 3: WO2004/088677 brochure
Patent document 4: JP-A-2006-286212
Patent document 5: JP-A-2007-200831
Patent document 6: JP-A-2001-110255
Non-patent document 1: D. P. Norton et al., Science vol. 274 (1996)755
To laminate different kinds of metals with favorable adhesion by a cold rolling method, it is necessary to bond different kinds of metals to each other by diffusion bonding (diffusion heat treatment) as the pretreatment and, thereafter, to apply cold rolling to the bonded metals. Although working efficiency of 90% or more is necessary to impart the high crystal orientation to an Ni layer after diffusion heat treatment, when strong rolling is applied to the different kinds of metals in a bonded state, due to the difference in mechanical properties between both materials, the difference in elongation occurs between the materials and hence, a large warp occurs. Accordingly, it is easy to expect that handling of materials becomes difficult in the manufacture of an elongated tape.
Further, in the above-mentioned clad material, materials to be bonded confine each other on a bonding boundary so that rolling is performed while causing the non-uniform deformation of the clad material whereby the uniform strain cannot be induced in the thickness direction. Further, the degree of roughness of the bonding boundary is also increased so that the thickness of the Ni layer in which crystals are oriented also becomes non-uniform. Accordingly, in the heat treatment after bonding, the stable manufacture of the substrate having the uniform and high crystal orientation in the longitudinal direction becomes difficult.
For example, a metal substrate disclosed in patent document 6 fails to set a crystal orientation rate of an Ni (200) surface parallel to a surface (assuming a diffraction peak intensity ratio of a (200) surface in θ/2θ measurement in X-ray diffraction as I(200)/ΣI(hkl)×100(%)) to high crystal orientation of 99% or more.
Further, even when a superconducting body layer is laminated to the metal substrate, crystal current density is approximately 105 A/cm2 so that crystal current density of high value on the order of 106 A/cm2 cannot be obtained.
Due to the above-mentioned reasons, at present, it is difficult to expect the further enhancement of superconductive property of elongated clad metal substrates which are proposed in patent documents 4, 5 and 6.
As other materials which exhibit the excellent crystal orientation property other than Ni, Cu and a Cu alloy can be named. Cu has characteristics that Cu is a face-centered cubic lattice and non-magnetic in the same manner as Ni, has the recrystallization temperature of approximately 200° C., and exhibits the crystal orientation at a low temperature. However, Cu is a low-strength material so that Cu has not been used positively.